feat: add create subaccount command

[paketeserrano]

Add 'account create' command for sub-account creation

Introduces 'upctl account create' to create UpCloud sub-accounts from the CLI.

What's new:

• Required flags: --username, --password
• Optional flags: --first-name, --last-name, --phone, --email, --timezone, --currency (defaults inherited from parent account), --allow-api (default: true), --allow-gui (default: false), --ip-filter (repeatable), --permission (repeatable, format target_type:target_identifier)
• Language is fixed to 'en' as no other values are accepted by the API
• Currency is validated client-side against EUR, GBP, USD, SGD
• Permission flags are validated client-side before any API call is made
• All access control arrays (roles, network_access, server_access, storage_access, tag_access) are sent as empty slices as required by the API
• Unit tests covering all flags, default inheritance, validation failures, and permission granting